Optoelectronic device comprising light-emitting diodes with improved light extraction

ABSTRACT

An optoelectronic device including a semiconductor substrate having a face, light-emitting diodes arranged on the face and including wired conical or frustoconical semiconductor elements, and an at least partially transparent dielectric layer covering the light-emitting diodes, the refractive index of the dielectric layer being between 1.6 et 1.8.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the national phase of International Application No.PCT/EP2014/077418, filed on Dec. 11, 2014, which claims the prioritybenefit of French patent application 13/63005, filed on Dec. 19, 2013,both of which applications are hereby incorporated by reference to themaximum extent allowable by law.

BACKGROUND

The present invention generally relates to optoelectronic devices basedon semiconductor materials and to methods for manufacturing the same.The present invention more specifically relates to optoelectronicdevices comprising light-emitting diodes formed by three-dimensionalelements, particularly semiconductor microwires or nanowires.

DISCUSSION OF THE RELATED ART

The term “optoelectronic devices with light-emitting diodes” designatesdevices capable of converting an electric signal into an electromagneticradiation, and particularly devices dedicated to the emission of anelectromagnetic radiation, particularly light. Examples ofthree-dimensional elements capable of forming light-emitting diodes aremicrowires or nanowires comprising a semiconductor material based on acompound mainly comprising at least one group-III element and onegroup-V element (for example, gallium nitride GaN), called III-Vcompound hereafter.

The extraction efficiency of an optoelectronic device is generallydefined by the ratio of the number of photons escaping from theoptoelectronic device to the number of photons emitted by thelight-emitting diodes. It is desirable for the extraction efficiency ofan optoelectronic device to be as high as possible.

A disadvantage of existing optoelectronic devices is that a fraction ofthe photons emitted within each light-emitting diode does not escapefrom the light-emitting diode.

Another disadvantage of existing optoelectronic devices is that aportion of the light emitted by each light-emitting diode is trapped orabsorbed by the neighboring light-emitting diodes.

SUMMARY

Thus, an object of an embodiment is to overcome at least part of thedisadvantages of previously-described optoelectronic devices withlight-emitting diodes, particularly with microwires or nanowires, and oftheir manufacturing methods.

Another object of an embodiment is to increase the extraction efficiencyof the optoelectronic device.

Another object of an embodiment is to decrease the proportion of lightwhich does not escape from each light-emitting diode.

Another object of an embodiment is to decrease the proportion of lightemitted by a light-emitting diode which is absorbed/trapped byneighboring light-emitting diodes.

Another object of an embodiment is for optoelectronic devices withlight-emitting diodes to be capable of being manufactured at anindustrial scale and at a low cost.

Thus, an embodiment provides an optoelectronic device comprising:

a semiconductor substrate comprising a surface;

light-emitting diodes supported by the surface and comprisingwire-shaped, conical, or tapered semiconductor elements; and

an at least partially transparent dielectric layer covering thelight-emitting diodes, the refractive index of the dielectric layerbeing in the range from 1.6 to 1.8.

According to an embodiment, the refractive index of the dielectric layeris in the range from 1.7 to 1.75.

According to an embodiment, each semiconductor element is mainly made ofa III-V compound.

According to an embodiment, each semiconductor element mainly comprisesgallium nitride.

According to an embodiment, the mean diameter of each semiconductorelement is in the range from 200 nm to 1 μm.

According to an embodiment, the encapsulation layer comprises a matrixmade of a first at least partially transparent material having particlesof a second material spread therein, the refractive index of the secondmaterial being greater than the refractive index of the first material.

According to an embodiment, the first material is a polysiloxane.

According to an embodiment, the second material is a dielectric materialselected from among titanium oxide (TiO2), zirconium oxide (ZrO2), andzinc sulfide (ZnS).

According to an embodiment, the encapsulation layer is made of amaterial selected from the group comprising epoxide polymers, siliconoxides of SiOx type, where x is a real number greater than 0 and smallerthan or equal to 2, silicon oxides of SiOyNz type, where y is a realnumber greater than 0 and smaller than or equal to 2 and z is greaterthan 0 and smaller than or equal to 0.57, and aluminum oxide (Al2O3).

According to an embodiment, the light-emitting diodes are distributed ona portion of the surface and the surface density of light-emittingdiodes on the portion decreases away from the edges of said portion.

According to an embodiment, the light-emitting diodes are distributed ona portion of the surface and the ratio of the perimeter of said portionto the surface area of said portion is greater than or equal to 4 for aunit surface area.

According to an embodiment, the portion corresponds to a surface with ahole.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages will be discussed indetail in the following non-limiting description of specific embodimentsin connection with the accompanying drawings, among which:

FIG. 1 is a partial simplified cross-section view of an embodiment of anoptoelectronic device with microwires or nanowires;

FIGS. 2 to 6 illustrate different configurations of paths followed bylight rays in microwires or nanowires;

FIG. 7 shows the variation of the distribution of the propagation modesof the light delivered by a light-emitting diodes with microwires ornanowires according to the refractive index of the material surroundingthe light-emitting diode;

FIG. 8 shows a curve of the variation of the proportion of trappedguided modes in a microwire or nanowire of a light-emitting diodeaccording to the refractive index of the material surrounding thelight-emitting diode;

FIG. 9 is a partial simplified top view of an optoelectronic devicecomprising light-emitting diodes with microwires or nanowires;

FIG. 10 shows the variations of the extraction efficiency according tothe considered position on the front surface of the optoelectronicdevice of FIG. 9; and

FIGS. 11 to 20 are partial simplified top views of embodiments ofoptoelectronic devices comprising light-emitting diodes with microwiresor nanowires.

DETAILED DESCRIPTION

For clarity, the same elements have been designated with the samereference numerals in the various drawings and, further, as usual in therepresentation of electronic circuits, the various drawings are not toscale. Further, only those elements which are useful to theunderstanding of the present disclosure have been shown and aredescribed. In particular, the means for biasing the light-emittingdiodes of an optoelectronic device are well known and will not bedescribed.

In the following description, unless otherwise indicated, the terms“substantially”, “approximately”, and “in the order of” mean “to within10%”. Further, “compound mainly formed of a material” or “compound basedon a material” means that a compound comprises a proportion greater thanor equal to 95% of said material, this proportion being preferentiallygreater than 99%.

The present description relates to optoelectronic devices withthree-dimensional elements, for example, wire-shaped, conical, ortapered elements, particularly microwires or nanowires.

The term “microwire” or “nanowire” designates a three-dimensionalstructure having an elongated shape along a preferential direction withat least two dimensions, called minor dimensions, in the range from 5 nmto 2.5 μm, preferably from 50 nm to 2.5 μm, the third dimension, calledmajor dimension, being at least equal to 1 time, preferably at least 5times, and more preferably still at least 10 times, the largest of theminor dimensions. In certain embodiments, the minor dimensions may besmaller than or equal to approximately 1 μm, preferably in the rangefrom 100 nm to 1 μm, more preferably from 100 nm to 800 nm. In certainembodiments, the height of each microwire or nanowire may be greaterthan or equal to 500 nm, preferably in the range from 1 μm to 50 μm.

In the following description, the term “wire” is used to mean “microwireor nanowire”. Preferably, the mean line of the wire which runs throughthe centers of gravity of the cross-sections, in planes perpendicular tothe preferential direction of the wire, is substantially rectilinear andis called “axis” of the wire hereafter.

FIG. 1 is a partial simplified cross-section view of an embodiment of anoptoelectronic device 5 with light-emitting diodes.

FIG. 1 shows a structure comprising, from bottom to top:

a first electrode 8;

a semiconductor substrate 10 comprising a lower surface 11 and an uppersurface 12, lower surface 11 being covered with first electrode 8 andupper surface 12 being preferably planar at least at the level of thelight-emitting diodes;

seed pads 16 made of a conductive material promoting the growth of wiresand arranged on surface 12;

wires 20 (six wires being shown) of height H1, each wire 20 being incontact with one of seed pads 16, each wire 20 comprising a lowerportion 22, of height H2, in contact with seed pad 16 and an upperportion 24, of height H3, continuing lower portion 22;

an insulating layer 26 extending on surface 12 of substrate 10 and onthe lateral sides of lower portion 22 of each wire 20;

a shell 28 comprising a stack of semiconductor layers covering eachupper portion 24;

a layer 30 forming a second electrode covering each shell 28 and furtherextending on insulating layer 26;

a conductive mirror layer 32 covering electrode layer 30 between wires20 without extending on wires 20; and

an encapsulation layer 34 covering the entire structure and particularlyelectrode 30 and comprising a front surface 36.

Optoelectronic device 5 may further comprise a layer of phosphors, notshown, provided on encapsulation layer 34 or confounded therewith.

The assembly formed by each wire 20, the associated seed pad 16, andshell 28 forms a light-emitting diode DEL. The base of diode DELcorresponds to seed pad 16. Shell 28 particularly comprises an activelayer which is the layer from which most of the electromagneticradiation delivered by light-emitting diode DEL is emitted.Light-emitting diodes DEL may be connected in parallel and form anassembly of light-emitting diodes. The assembly may comprise from a fewlight-emitting diodes DEL to one thousand light-emitting diodes.

It is possible for light-emitting diodes DEL not to be formed over theentire surface 12. The portion of surface 12 having light-emittingdiodes formed thereon is called the active area.

Substrate 10 may correspond to a solid structure or correspond to alayer covering a support made of another material. Substrate 10 ispreferably a semiconductor substrate, for example, a substrate made ofsilicon, of germanium, of silicon carbide, of a III-V compound, such asGaN or GaAs, or a ZnO substrate. Preferably, substrate 10 is asingle-crystal silicon substrate. Preferably, it is a semiconductorsubstrate compatible with manufacturing methods implemented inmicroelectronics. Substrate 10 may correspond to a multilayer structureof silicon on insulator type, SOI.

The substrate may be heavily doped, lightly doped or non-doped. In thecase where the substrate is heavily doped, semiconductor substrate 10may be doped to lower the electric resistivity down to a resistivityclose to that of metals, preferably lower than a few mohm·cm. Substrate10 for example is a heavily-doped substrate having a dopantconcentration in the range from 5*1016 atoms/cm3 to 2*1020 atoms/cm3. Inthe case where the substrate is lightly doped, for example, with adopant concentration smaller than or equal to 5*1016 atoms/cm3,preferably substantially equal to 1015 atoms/cm3, a doped region of thefirst conductivity type or of a second conductivity type, opposite tothe first type, more heavily doped than the substrate, may be provided,which extends in substrate 10 from surface 12 under seed pads 16. In thecase of a silicon substrate 10, examples of P-type dopants are boron (B)or indium (In) and examples of N-type dopants are phosphorus (P),arsenic (As), or antimony (Sb).

Surface 12 of silicon substrate 10 may be a (100) surface.

Seed pads 16, also called seed islands, are made of a material promotingthe growth of wires 20. A treatment may be provided to protect thelateral sides of the seed pads and the surface of the substrate portionswhich are not covered with the seed pads to prevent the wires fromgrowing on the lateral sides of the seed pads and on the surface of thesubstrate portions which are not covered with the seed pads. Thetreatment may comprise forming a dielectric region on the lateral sidesof the seed pads and extending on top of and/or inside of the substrate,with no wire growth on the dielectric region. As a variation, seed pads16 may be replaced with a seed layer covering surface 12 of substrate10. A dielectric region may then be formed above the seed layer toprevent the growth of wires in unwanted areas.

As an example, the material forming seed pads 16 may be a nitride, acarbide, or a boride of a transition metal from column IV, V, or VI ofthe periodic table of elements or a combination of these compounds. Asan example, seed pads 16 may be made of aluminum nitride (AlN), boron(B), boron nitride (BN), titanium (Ti), titanium nitride (TiN), tantalum(Ta), tantalum nitride (TaN), hafnium (Hf), hafnium nitride (HfN),niobium (Nb), niobium nitride (NbN), zirconium (Zr), zirconium borate(ZrB2), zirconium nitride (ZrN), silicon carbide (SiC), tantalumcarbo-nitride (TaCN), magnesium nitride in MgxNy form, where x isapproximately equal to 3 and y is approximately equal to 2, for example,magnesium nitride in Mg3N2 form or magnesium gallium nitride (MgGaN),tungsten (W), tungsten nitride (WN), or a combination thereof.

Seed pads 16 may be doped with the same conductivity type as substrate10.

Insulating layer 26 may be made of a dielectric material, for example,of silicon oxide (SiO2), silicon nitride (SixNy, where x isapproximately equal to 3 and y is approximately equal to 4, for example,Si3N4), silicon oxynitride (particularly of general formula SiOxNy, forexample, Si2ON2), aluminum oxide (Al2O3), hafnium oxide (HfO2), ordiamond. As an example, the thickness of insulating layer 26 is in therange from 5 nm to 500 nm, for example, equal to approximately 30 nm.

Wires 20 are at least partly formed from at least one semiconductormaterial. Wires 20 may be at least partly formed from semiconductormaterials mainly comprising a III-V compound, for example, a III-Ncompound. Examples of group-III elements comprise gallium (Ga), indium(In), or aluminum (Al). Examples of III-N compounds are GaN, AN, InN,InGaN, AlGaN, or AlInGaN. Other group-V elements may also be used, forexample, phosphorus or arsenic. Generally, the elements in the III-Vcompound may be combined with different molar fractions.

Wires 20 may comprise a dopant. As an example, for III-V compounds, thedopant may be selected from the group comprising a group-II P-typedopant, for example, magnesium (Mg), zinc (Zn), cadmium (Cd), or mercury(Hg), a group-IV P-type dopant, for example, carbon (C), or a group-IVN-type dopant, for example, silicon (Si), germanium (Ge), selenium (Se),sulfur (S), terbium (Tb), or tin (Sn).

The cross-section of wires 20 may have different shapes, such as, forexample, a shape which may be oval, circular, or polygonal, particularlytriangular, rectangular, square, or hexagonal. It should thus beunderstood that the term “diameter” mentioned in relation with across-section of a wire or of a layer deposited on this wire designatesa quantity associated with the surface area of the targeted structure inthis cross-section, corresponding, for example, to the diameter of thedisk having the same surface area as the wire cross-section. Height H1of each wire 20 may be in the range from 250 nm to 50 μm. Each wire 20may have an elongated semiconductor structure along an axissubstantially perpendicular to surface 12. Each wire 20 may have ageneral cylindrical shape. The axes of two wires 20 may be distant byfrom 0.5 μm to 10 μm, and preferably from 1.5 μm to 6 μm. As an example,wires 20 may be regularly distributed, particularly in a hexagonalnetwork.

As an example, lower portion 22 of each wire 20 is mainly formed of theIII-N compound, for example, gallium nitride, of same doping type asregion 14, for example, type N, for example, silicon-doped. Lowerportion 22 extends along a height H2 which may be in the range from 100nm to 25 μm.

As an example, upper portion 24 of each wire 20 is at least partiallymade of a III-N compound, for example, GaN. Upper portion 24 may beN-type doped, possibly less heavily doped than lower portion 22, or notbe intentionally doped. Upper portion 24 extends along a height H3 whichmay be in the range from 100 nm to 25 μm.

Shell 28 may comprise a stack of a plurality of layers, particularlycomprising:

-   -   an active layer covering upper portion 24 of the associated wire        20;    -   an intermediate layer having a conductivity type opposite to        that of lower portion 22 covering the active layer; and    -   a bonding layer covering the intermediate layer and covered with        electrode 30.

The active layer is the layer from which most of the radiation deliveredby light-emitting diode DEL is emitted. According to an example, theactive layer may comprise confinement means, such as multiple quantumwells. It is for example formed of an alternation of GaN and of InGaNlayers having respective thicknesses from 5 to 20 nm (for example, 8 nm)and from 1 to 10 nm (for example, 2.5 nm). The GaN layers may be doped,for example of type N or P. According to another example, the activelayer may comprise a single InGaN layer, for example, having a thicknessgreater than 10 nm.

The intermediate layer, for example, P-type doped, may correspond to asemiconductor layer or to a stack of semiconductor layers and allows theforming of a P-N or P-I-N junction, the active layer being comprisedbetween the intermediate P-type layer and upper N-type portion 24 of theP-N or P-I-N junction.

The bonding layer may correspond to a semiconductor layer or to a stackof semiconductor layers and enables to form an ohmic contact between theintermediate layer and electrode 30. As an example, the bonding layermay be very heavily doped, of a type opposite to that of lower portion22 of each wire 20, until degeneration of the semiconductor layer(s),for example, P-type doped at a concentration greater than or equal to1020 atoms/cm3.

The stack of semiconductor layers may comprise an electron barrier layerformed of a ternary alloy, for example, made of aluminum gallium nitride(AlGaN) or of aluminum indium nitride (AlInN) in contact with the activelayer and the intermediate layer, to provide a good distribution ofelectric carriers in the active layer.

Electrode 30 is capable of biasing the active layer of each wire 20 andof letting through the electromagnetic radiation emitted bylight-emitting diodes DEL. The material forming electrode 30 may be atransparent and conductive material such as indium tin oxide (ITO),aluminum-doped zinc oxide, or graphene. As an example, electrode layer30 has a thickness in the range from 5 nm to 200 nm, preferably from 20nm to 50 nm.

Conductive mirror layer 32 preferably corresponds to a metal layer, forexample, made of aluminum, silver, copper, or zinc. As an example,conductive mirror layer 32 has a thickness in the range from 20 nm to300 nm, preferably from 100 nm to 200 nm.

Encapsulation layer 34 is made of an at least partially transparentinsulating material. The maximum thickness of encapsulation layer 34 isgreater than height H1 of wires 20. Encapsulation layer 34 extendsbetween wires 20 and covers each wire 20. The space between wires 20 istotally filled with encapsulation layer 34. The maximum thickness ofencapsulation layer 34 is in the range from 250 nm to 50 μm so thatencapsulation layer 34 fully covers electrode 30 at the top oflight-emitting diodes DEL.

The active layer of shell 28 of each light-emitting diode DEL emitslight in all directions.

FIGS. 2 to 6 illustrate the paths traveled by light rays R for differentlight emission configurations. In FIGS. 2 to 6, layers 30, 32, and 34have not been shown. Call 0 the angle formed by light ray R relative todirection D perpendicular to the lateral walls of wire 20 and θC thecritical total reflection angle of the assembly comprising wire 20 andthe active layer of shell 28.

According to emission angle θ, the light emitted by the active layer ofshell 28 may either couple to a radiated mode called RL, as illustratedin FIG. 2, or to a guided mode called GL, as illustrated in FIG. 3.Guided modes GL have an emission angle θ greater than critical totalreflection angle θC and propagating in zigzag along wire 20. Conversely,radiated modes RL have an emission angle θ smaller than critical totalreflection angle θC and are totally transmitted in encapsulation layer34.

Critical total reflection angle θC is provided by Snell's law accordingto the following relation (1):θC=a sin(nencap/nwire)  (1)

where nencap is the real part of the optical refractive index ofencapsulation layer 34 and nwire is the real part of the opticalrefractive index of wire 20 and of its shell 28, nwire being greaterthan nencap.

The optical refractive index is a dimensionless number whichcharacterizes the optical properties of a medium, particularly theabsorption and the diffusion. The refractive index is equal to the realpart of the complex optical index. The refractive index may for examplebe determined by ellipsometry.

Under angle of incidence θ, guided modes GL break up into modes lost inthe substrate SGL (FIG. 4), into reflected modes RGL (FIG. 5) andtransmitted modes TGL (FIG. 6). Modes SGL are guided to the base of wire20 and lost in substrate 10. Modes TGL have an angle of incidence at theupper face of wire 20 which is smaller than the critical totalreflection angle and are thus transmitted in encapsulation layer 34.Modes RGL are in total reflection condition at the upper face and aresent back towards the base of wire 20 without being extracted.

Among guided modes GL, only transmitted guided modes TGL take part inthe light perceived by an observer. The modes guided towards thesubstrate, SGL, are directly lost and reflected modes RGL remain trappedinside of wire 20 until they are absorbed or lost in substrate 10.

FIG. 7 shows the proportion of modes RL, GL, SGL, RGL, and TGL accordingto refractive index nencap of encapsulation layer 34. The proportion oflight extracted from the light-emitting diode is the sum of proportionsRL and TGL.

FIG. 8 shows a variation curve CRGL corresponding to the proportion ofguided modes RGL relative to the total number of guided modes GLaccording to refractive index nencap of encapsulation layer 34. Thecurves of FIGS. 7 and 8 have been obtained in the case of a GaN wire ofhexagonal cross-section having a mean diameter of 800 nm and a shellhaving a 275-nm thickness.

The proportion of radiated modes, RL, increases while the proportion ofguided modes, GL, decreases as the refractive index of encapsulationlayer 34 increases. Further, the proportion of trapped guided light RGLdecreases to become zero for a refractive index of encapsulation layer34 equal to approximately 1.73.

By simulation, the inventors have shown that the shape of the curves ofvariation of the proportions of modes RL, GL, SGL, RGL, and TGL issubstantially the same independently from the mean diameter of wire 20,as soon as the mean diameter of wire 20 is greater than 200 nm. Inparticular, the refractive index at which propagation mode RGL cancelsis substantially independent from the mean diameter of wire 20, as soonas the mean diameter of wire 20 is greater than 200 nm.

To be perceived by an observer watching optoelectronic device 5, thelight should leave encapsulation layer through front surface 36. Surface36 may correspond to a free surface, that is, in contact with air. Thegreater the difference between the refractive index of encapsulationlayer 34 and the index of air, the lower the critical total reflectionangle, measured relative to a direction perpendicular to surface 36,that is, the more the light originating from light-emitting diodes DELtends to reflect on surface 36. It is thus not desirable for therefractive index of encapsulation layer 34 to be too high.

When the material forming the wires and the active layers is a III-Vcompound, the inventors have shown by simulation that the bestcompromise can be obtained with a refractive index of encapsulationlayer 34 in the range from 1.7 to 1.75, preferably in the range from1.72 to 1.74, more preferably of approximately 1.73.

Preferably, the mean diameter of wire 20 is in the range from 200 nm to1 μm, preferably from 300 nm to 800 nm.

Encapsulation layer 34 may comprise a matrix of an at least partiallytransparent inorganic material having particles of a dielectric materialpossibly spread therein. The refractive index of the dielectric materialforming the particles is greater than the refractive index of thematerial forming the matrix. According to an example, encapsulationlayer 34 comprises a matrix made of silicone, also called polysiloxane,and further comprises particles of a dielectric material spread in thematrix. The particles are made of any type of material providingrelatively spherical nanometer-range particles having an adaptedrefractive index. As an example, the particles may be made of titaniumoxide (TiO2), zirconium oxide (ZrO2), zinc sulfide (ZnS), lead sulfide(PbS), or amorphous silicon (Si). The mean diameter of a is defined asbeing the diameter of the sphere of same volume. The mean diameter ofthe particles of the dielectric material is in the range from 2 nm to250 nm. The volume concentration of particles with respect to the totalweight of encapsulation layer 34 is in the range from 1% to 50%.

According to another example, the inorganic material is selected fromthe group comprising epoxide polymers, silicon oxides of SiOx type,where x is a real number greater than 0 and smaller than or equal to 2,silicon oxides of SiOyNz type, where y is a real number greater than 0and smaller than or equal to 2 and z is greater than 0 and smaller thanor equal to 0.57, and aluminum oxide (Al2O3).

Encapsulation layer 34 may be made of an at least partially transparentorganic material. According to an example, encapsulation layer 34 ismade of polyimide. According to another example, encapsulation layer 34is made of an epoxide polymer which further comprises particles of adielectric material distributed in the matrix. The particles may be madeof titanium oxide (TiO2), zirconium oxide (ZrO2), zinc sulfide (ZnS),lead sulfide (PbS), or amorphous silicon (Si).

To improve the extraction efficiency of optoelectronic device 5, asurface treatment, called texturing, may be applied to surface 36 ofencapsulation layer 34 to form raised areas on surface 36. For anencapsulation layer 34 made of an inorganic material, the method oftexturing surface 36 may comprise a step of chemical etching or a stepof mechanical abrasion, possibly in the presence of a mask protectingportions of surface 36 treated to promote the forming of patterns at thesurface. For a layer 34 made of an organic material, the method oftexturing surface 36 may comprise a step of embossing, moulding, etc.

To improve the extraction efficiency of optoelectronic device 5,encapsulation layer 34 may be covered with an at least partiallytransparent additional layer. The refractive index of the additionallayer is then between the refractive index of encapsulation layer 34 andthe refractive index of air. As a variation, a stack of at least twolayers may cover encapsulation layer 34. The refractive indexes of thelayers in the stack decrease from the first layer of the stack incontact with encapsulation layer 34 to the last layer of the stack incontact with air, the refractive index of the first layer being smallerthan the refractive index of encapsulation layer 34 and the refractiveindex of the last layer being greater than the refractive index of air.

The optoelectronic device according to the previously-describedembodiment enables to advantageously increase the general extractionefficiency of the optoelectronic device, that is, the efficiencymeasured over the entire surface 36.

The extraction efficiency may be measured locally, that is, for aportion of surface 36. It then corresponds to the ratio of the quantityof light which escapes from the optoelectronic device through theconsidered portion to the quantity of light delivered by thelight-emitting diodes of this portion. It is desirable for thevariations of the local extraction efficiency over the entire surface 36to be as low as possible to avoid for an observer to perceive luminancedifferences when watching optoelectronic device 5.

FIG. 9 is a top view of an example of optoelectronic device 50,comprising all the elements of optoelectronic device 5 shown in FIG. 1and having light-emitting diodes DEL regularly distributed therein, forexample, in rows and in columns, on a square active area 51. The lateraledges of active area 51 are designated with reference numeral 52 and thecorners of active area 51 are designated with reference numeral 54. Eachlight-emitting diode is schematically shown as a point. As an example,except for the diodes located along edges 52, each light-emitting diodeDEL is located at the center of a square comprising a light-emittingdiode at each apex and a light-emitting diode in the middle of eachedge.

In the example shown in FIG. 9, the density of light-emitting diodes persurface area unit is substantially constant over the entire active area51. As an example, the surface density of light-emitting diodes issubstantially constant and in the range from 4*106/cm2 to 3*107/cm2.

FIG. 10 shows the variation of the local extraction efficiency ofoptoelectronic device 50 of FIG. 9 over one quarter of active area 51.The curve of FIG. 10 has been obtained in the case of an array of GaNnanowires of hexagonal cross-section, the distance between the axes oftwo nanowires being 3 times the mean radius of shell 28 and therefractive index of the material of encapsulation layer 34 being equalto 1.75.

The local extraction efficiency is greater along edges 52 than at thecenter of active area 51. Further, the local extraction efficiency isgreater at apexes 54 than on edges 52 of active area 51. The explanationof this phenomenon is that the larger the number of close neighbors of alight-emitting diode, the higher the probability for the light raysemitted by this light-emitting diode to hit one of the neighboringlight-emitting diodes and to be absorbed or trapped by said neighbors.

By simulation, the inventors have shown a decrease in the extractionefficiency as soon as the distance between the axes of two adjacentlight-emitting diodes is smaller than 15 times the mean radius of shell28.

For distances between the axes of adjacent light-emitting diodes smallerthan 15 times the mean radius of shell 28, it has been observed that theextraction efficiency of the center of active area 51 reaches a minimumvalue independent from the number of rows and columns when the number ofrows and columns is greater than approximately 50.

FIG. 11 is a view similar to FIG. 9 of an embodiment of anoptoelectronic device 60. Optoelectronic device 60 comprises all theelements of optoelectronic device 50, with the difference that thedensity of light-emitting diodes per surface area unit graduallyincreases from the center of the device all the way to edges 52. Moreparticularly, the surface density of light-emitting diodes at the centerof active area 51 is smaller than the surface density of light-emittingdiodes along edges 52. Further, the surface density of light-emittingdiodes along edges 52 is smaller than the surface density oflight-emitting diodes at apexes 54 of active area 51.

As an example, the variation of the surface density of light-emittingdiodes may correspond to the inverse of the variation of the extractionefficiency such as shown in FIG. 10. As an example, the surface densityof light-emitting diodes at the center of the active area of theoptoelectronic device may be in the range from 2*106/cm2 to 6*106/cm2while the surface density of light-emitting diodes along an edge of theactive area of the optoelectronic device may be in the range from7*106/cm2 to 2*107/cm2.

According to another embodiment, the inventors have shown that theuniformity of the extraction efficiency can be improved by increasingthe ratio of the perimeter of the active area to the surface of theactive area. Preferably, ratio P/A of the perimeter to the surface ofthe active area is greater than 4 for a unit active surface area,preferably greater than or equal to 4.5, more preferably greater than orequal to 5, and particularly greater than or equal to 6.

FIGS. 12 to 20 show simplified top views of embodiments ofoptoelectronic devices for each of which only the contour of the activearea has been shown. For each of these examples, the ratio of theperimeter of the active area to the surface area of the active area isgreater than that obtained for a square of same surface area.

In FIG. 12, active area 70 has a ring shape comprising a square externaledge 72 and a square internal edge 74. In FIG. 13, active area 76comprises one or more than one rectangular area 78, two rectangularareas being shown. In FIG. 14, active area 80 comprises one or more thanone strip 82 with wavy edges, two strips 82 being shown. In FIG. 15,active area 84 has a triangular shape. In FIG. 16, active area 86 isstar-shaped. In FIG. 17, active area 88 comprises a star-shaped outeredge 90 and a star-shaped inner edge 91. Advantageously, the outerperimeter, and possibly the inner perimeter, of the active area follow acurve which is close to a fractal curve. In FIGS. 18, 19, and 20, activeareas 94, 96, and 98 respectively have the shape of a Koch snowflakeafter two, three, or four iterations. Ratio P/A for a unit surface areaof the active area is respectively 6.4, 8.5, and 11.4 for active areas94, 96, and 98. FIGS. 12 and 17 show examples of active areascorresponding to surfaces with holes.

An embodiment of a manufacturing method providing optoelectronic device5 comprises the steps of:

(1) Forming, on surface 12 of substrate 10, seed pads 16.

Seed pads 16 may be obtained by depositing a seed layer on surface 12and by etching portions of the seed layer all the way to surface 12 ofsubstrate 10 to delimit the seed pads. The seed layer may be depositedby a method such as chemical vapor deposition (CVD) or metal-organicchemical vapor deposition (MOCVD), also known as metal-organic vaporphase epitaxy (MOVPE). However, methods such as molecular-beam epitaxy(MBE), gas-source MBE (GSMBE), metal-organic MBE (MOMBE),plasma-assisted MBE (PAMBE), atomic layer epitaxy (ALE), hydride vaporphase epitaxy (HVPE) may be used, as well as atomic layer deposition(ALD). Further, methods such as evaporation or reactive cathodesputtering may be used.

When seed pads 16 are made of aluminum nitride, they may besubstantially textured and have a preferred polarity. The texturing ofpads 16 may be obtained by an additional treatment performed after thedeposition of the seed layer. It for example is an anneal under anammonia flow (NH3).

(2) Protecting the portions of surface 12 of substrate 10 which are notcovered with seed pads 16 to avoid the subsequent growth of wires onthese portions. This may be obtained by a nitriding step which causesthe forming, at the surface of substrate 10, between seed pads 16, ofsilicon nitride regions (for example, SiN or Si3N4). This may also beobtained by a step of masking substrate 10 between seed pads 16,including the deposition of a layer, for example of a SiO2 or SiN orSi3N4 dielectric, and then the etching of this layer outside of seedpads 16 after a photolithography step. In this case, the masking layermay extend over seed pads 16. When the protection step (2) is carriedout by a step of masking substrate 10, the seed layer etch step may beavoided. Seed pads 16 are then formed of a uniform continuous layerhaving its surface left free where the wires cross.

(3) Growing lower portion 22 of each wire 20 along height H2. Each wire20 grows from the top of the underlying seed pad 16.

Wires 20 may be grown by a process of CVD, MOCVD, MBE, GSMBE, PAMBE,ALE, HVPE, ALD type. Further, electrochemical processes may be used, forexample, chemical bath deposition (CBD), hydrothermal processes, liquidaerosol pyrolysis, or electrodeposition.

As an example, the wire growth method may comprise injecting into areactor a precursor of a group-III element and a precursor of a group-Velement. Examples of precursors of group-III elements aretrimethylgallium (TMGa), triethylgallium (TEGa), trimethylindium (TMIn),or trimethylaluminum (TMAl). Examples of precursors of group-V elementsare ammonia (NH3), tertiarybutylphosphine (TBP), arsine (AsH3), orunsymmetrical dimethylhydrazine (UDMH).

According to an embodiment of the invention, in a first phase of growthof the wires of the III-V compound, a precursor of an additional elementis added in excess, in addition to the precursors of the III-V compound.The additional element may be silicon (Si). An example of a precursor ofsilicon is silane (SiH4).

The presence of silane among precursor gases causes the incorporation ofsilicon within the GaN compound. A lower N-type doped portion 22 is thusobtained. This further translates as the forming of a silicon nitridelayer, not shown, which covers the periphery of portion 22 of height H2,except for the top, as portion 22 grows.

(4) Growing upper portion 24 of height H3 of each wire 20 on the top oflower portion 22. For the growth of upper portion 24, thepreviously-described operating conditions of the MOCVD reactor are, asan example, maintained but for the fact that the silane flow in thereactor is decreased, for example, by a factor greater than or equal to10, or stopped. Even when the silane flow is stopped, upper portion 24may be N-type doped due to the diffusion in this active portion ofdopants originating from the adjacent passivated portion or due to theresidual doping of GaN.

(5) Forming insulating layer 26, for example, by conformal deposition ofan insulating layer over the entire structure obtained at step (4) andetching this layer to expose upper portion 24 of each wire 20.

(6) Forming by epitaxy, for each wire 20, the layers forming shell 28.Given the presence of insulating layer 26 covering the periphery oflower portion 22, the deposition of the layers forming shell 28 onlyoccurs on the upper portion 24 of wire 20 which is not covered withinsulating layer 26;

(7) Forming electrode 30, for example, by conformal deposition;

(8) Forming conductive mirror layer 32 for example by physical vapordeposition (PVD) over the entire structure obtained at step (7) or forexample by evaporation or by cathode sputtering and etching of thislayer to expose each wire 20;

(9) Forming encapsulation layer 34. When encapsulation layer 34 is madeof silicone, encapsulation layer 34 may be deposited by a spin coatingdeposition method, by an inkjet printing method, or by a silk-screeningmethod. When encapsulation layer 34 is an oxide, it may be deposited byCVD; and

(10) Sawing substrate 10 to separate the optoelectronic devices.

In the previously-described embodiment, insulating layer 26 covers theentire periphery of lower portion 22 of each wire 20. As a variation, itis possible for a portion of lower portion 22 not to be covered withinsulating layer 26. In this case, insulating layer 26 covers wire 20 upto a height smaller than H2 and shell 28 covers wire 20 up to a heightgreater than H3. It is possible for layer 26 not to cover lower portion22 of each wire 20. In this case, shell 28 may cover each wire 20 up toheight H1.

In the previously-described embodiment, insulating layer 26 does notcover the periphery of upper portion 24 of each wire 20. As a variation,insulating layer 26 may cover a portion of upper portion 24 of each wire20. In this case, insulating layer 26 covers wire 20 up to a heightgreater than H2 and shell 28 covers wire 20 up to a height smaller thanH3.

According to another variation, insulating layer 26 may, for each wire20, partially cover the lower portion of shell 30.

According to a variation of the previously-described manufacturingmethod, the layers forming shell 28 may be formed before insulatinglayer 26 over the entire wire 20 or only over a portion of wire 20, forexample, upper portion 24.

Specific embodiments of the present invention have been described.Various alterations and modifications will occur to those skilled in theart. Further, although, in the previously-described embodiments, eachwire 20 comprises a passivated portion 22, at the base of the wire incontact with one of seed pads 16, passivated portion 22 may be absent.

Further, although embodiments have been described for an optoelectronicdevice for which shell 28 covers the top of the associated wire 20 and aportion of the lateral sides of wire 20, it is possible to only providethe shell at the top of wire 20.

Various embodiments with different variations have been describedhereabove. It should be noted that those skilled in the art may combinevarious elements of these various embodiments and variations withoutshowing any inventive step. In particular, optoelectronic devicecomprising an encapsulation layer having a refractive index in the rangefrom 1.7 to 1.75 may, further, comprise a surface density oflight-emitting diodes which is variable, as for example shown in FIG.11. Further, the optoelectronic device comprising an encapsulation layerhaving a refractive index in the range from 1.7 to 1.75 may further havea ratio of the perimeter to the surface area of the active area which isgreater than the ratio obtained for a square active area, for example aspreviously described in relation with FIGS. 12 to 20. Further, theoptoelectronic device having a ratio of the perimeter to the surface ofthe active area which is greater than the ratio obtained for a squareactive area may further comprise a variable surface density oflight-emitting diodes.

What is claimed:
 1. An optoelectronic device comprising: a semiconductorsubstrate having a surface; light-emitting diodes supported by thesurface comprising wire-shaped, conical, or tapered semiconductorelements; and an at least partially transparent dielectric layercovering the light-emitting diodes, the refractive index of thedielectric layer being in the range from 1.6 to 1.8, the maximumthickness of the dielectric layer being in the range from 250 nm to 50μm.
 2. The optoelectronic device of claim 1, wherein the refractiveindex of the dielectric layer is in the range from 1.7 to 1.75.
 3. Theoptoelectronic device of claim 1, wherein each semiconductor element ismainly made of a III-V compound.
 4. The optoelectronic device of claim3, wherein each semiconductor element mainly comprises gallium nitride.5. The optoelectronic device of claim 1, wherein the mean diameter ofeach semiconductor element is in the range from 200 nm to 1 μm.
 6. Theoptoelectronic device of claim 1, wherein the dielectric layer comprisesa matrix made of a first at least partially transparent material havingparticles of a second material spread therein, the refractive index ofthe second material being greater than the refractive index of the firstmaterial.
 7. The optoelectronic device of claim 6, wherein the firstmaterial is a polysiloxane.
 8. The optoelectronic device of claim 6,wherein the second material is a dielectric material selected from amongtitanium oxide, zirconium oxide, and zinc sulfide.
 9. The optoelectronicdevice of claim 1, wherein the dielectric layer is made of a materialselected from the group comprising epoxide polymers, silicon oxides ofSiOx type, where x is a real number greater than 0 and smaller than orequal to 2, silicon oxides of SiOyNz type, where y is a real numbergreater than 0 and smaller than or equal to 2 and z is greater than 0and smaller than or equal to 0.57, and aluminum oxide.
 10. Theoptoelectronic device of claim 1, wherein the light-emitting diodes aredistributed over a portion of the surface and the surface density oflight-emitting diodes on the portion decreases away from the edges ofsaid portion.
 11. The optoelectronic device of claim 1, wherein thelight-emitting diodes are distributed over a portion of the surface andwherein the ratio of the perimeter of said portion to the surface areaof said portion is greater than or equal to 4 for a unit surface area.12. The optoelectronic device of claim 11, wherein the portioncorresponds to a surface with a hole.